[doc,patch] Document that -fwhole-program isn't supported for Fortran

[François-Xavier Coudert]

For Fortran, -fwhole-program isn't usable because the Fortran
front-end issues more than one decl per function, thus making the call
graph used in this optimisation wrong. This patch documents that fact
in gcc/doc/invoke.texi (a one-liner) and catches it in the front-end
(issuing an error when gfortran is called with -fwhole-program).

Patch was regtested on x86_64-linux, including make pdf, info and
html. Do I need approval for this one-line patch to the doc? I don't
think so, and intend to commit it in 48 hours unless someone objects.

FX
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